If you add a virtual server with a range of ports over the entire thing, but put 0-65535 instead of 1-65535, it will accept it when you hit the "+" button, but not add it to the table, or report an error. In addition, if you don't fill out all the port fields correctly, it will pop up an error message box saying you have to put in a port number in the 0-65535 range. That might be better phrased as 1-65535, as I've never actually seen a production use of port 0.